Let us consider a typical condition for estimating the value of mean photon number in the focused beam illuminating the object in experiment. For instance, if it is given an incident beam with \(\lambda = 694\)  nm, optical power of 1 mW and observation time of 1 ms, the focused beam then has an optical power of \(0.01\)  mW (where it has been assumed that there is \(1/100\)  of total incident power in the focused mode [1]}). Finally, the mean photon number of the focused beam can be worked out as \(\langle \hat{n}^{\rm {w}}_{b} \rangle \simeq 3.47 \times 10^{10}\) .
